The present invention relates generally to a viscous fluid heater that generates heat by rotating a rotor in a heating chamber containing viscous fluid. More particularly, the present invention pertains to a viscous fluid heater capable of controlling heating performance.
An automotive vehicle is generally provided with a hot-water type heater. In a vehicle having such a heater, coolant is used to cool the engine. The coolant is heated by engine heat. The heater typically has a heater core housed in a duct. The heated coolant is sent to the heater core. This heats the air sent to the passenger compartment and warms the passenger compartment.
In a diesel engine vehicle or a lean burn engine vehicle, the amount of heat produced by the engine is relatively small. Thus, the amount of heat transmitted to the coolant is small. It is difficult for the coolant to reach a certain temperature such as 80.degree. C. when the amount of heat sent to the heater core is small. Therefore, the heat used to warm the passenger compartment may be insufficient.
To solve this problem, a viscous fluid heater arranged in an engine coolant circulating circuit has been proposed. The viscous fluid heater is used for heating the coolant and includes a housing, which houses a heating chamber and a water jacket (heat exchange chamber). The heater also has a drive shaft and a rotor that are driven by the engine. Viscous fluid (such as high viscosity silicone oil) is contained in the heating chamber and is sheared by the rotor. This causes fluid friction and generates heat. The heat raises the temperature of the fluid (engine coolant) circulating through the water jacket.
The temperature of the viscous fluid contained in the heating chamber increases as the engine speed increases, regardless of the temperature of the circulating fluid flowing through the circulating circuit. When the high viscosity silicone oil is heated to a temperature of, for example, 250.degree. C. or higher, the silicone oil becomes vulnerable to thermal deterioration caused by the heat and mechanical deterioration caused by the shearing. Such deterioration decreases the heating efficiency during the shearing. As a result, the heating performance of the heater, which is used to warm the passenger compartment, is degraded.
To prevent deterioration of the silicone oil, a heater that varies its heating performance has been proposed. U.S. Pat. No. 4,974,778 describes such a heater. When the passenger compartment is sufficiently warm, the heating performance is lowered to stop unnecessary shearing of the silicone oil with the rotor.
The heating performance is varied by moving the silicone oil between a heating chamber and a reservoir chamber to adjust the amount of silicone oil in the heating chamber. However, the high viscosity of the silicone oil slows the movement of the silicone oil from the reservoir chamber to the heating chamber, especially when adjusting the heating performance under lower temperatures. Such slow movement of the silicone oil interferes with the smooth charging of the silicone oil into the heating chamber. This degrades the responsiveness of the heater when varying the heating performance.